The present invention relates to processes, materials and devices for plasma etching of Sixe2x80x94Ge layers for fabricating optically smooth Sixe2x80x94Ge surfaces, and particularly to fabricating waveguides in opto-electronic integrated circuits employing Sixe2x80x94Ge.
A waveguide is a conductor or directional transmitter for electromagnetic waves. Waveguides are, for example employed in opto-electronic integrated circuits. An opto-electronic integrated circuit (OEIC) device combines optics with electronics in an integrated form. OEIC technology is commonly used for example in optical fiber communication devices and methods. A typical OEIC includes conventional IC (integrated circuit) components as well as optical components. Conventional IC components include for example, transistors, diodes, resistors and electrically conductive interconnects. Examples of optical components include light receiving devices such as photodiodes, light emitting devices such as light emitting diodes (LED), optical reflectors such as metallic mirrors, optical filters and waveguides.
Typical OEIC waveguides are optical interconnects that provide an optical path between optical and/or opto-electronic components. Conventional waveguide materials that are employed in OEIC devices include monocrystalline silicon. Typically, an OEIC waveguide is embedded within sidewalls and top and bottom claddings. It is recognized that roughness of waveguide sidewall surfaces, and roughness of the surfaces of top and/or bottom claddings, results in light scattering that causes a significant light propagation loss when light is transmitted through the waveguide. It is therefore highly desirable to provide sidewall and cladding surfaces adjacent the waveguide that exhibit very low surface roughness.
Also, it is desirable to employ a waveguide sidewall material that is similar to the waveguide material in chemical and physical properties, particularly including mechanical and thermal properties, in order to maximize the reliability and durability of the waveguide structure in the OEIC device.
Desirably, techniques for fabricating OEIC waveguides should employ relatively low fabricating temperatures in order to limit, or avoid if possible, heat caused damage or degradation of other components of the OEIC structure such as a semiconductor wafer.
Sixe2x80x94Ge (silicon-germanium, also known as germanium doped silicon) is a suitable material for waveguides. Particularly when an Sixe2x80x94Ge waveguide is enclosed within Sixe2x80x94Ge sidewalls such that the Sixe2x80x94Ge waveguide material has a higher refractive index than the refractive index of the Sixe2x80x94Ge sidewall material. However, conventional Sixe2x80x94Ge etch techniques using etch chemistries such as HBr/Cl2, result in Sixe2x80x94Ge sidewall roughness that causes a high level of light scattering. Also, these conventional etch chemistries exhibit a relatively low etch selectivity to organic photoresist; this low selectivity typically ranges from about 2-3:1. It has been found that this low selectivity is unsuitable for successfully etching the typical requirements of 1.5-7 xcexcm Sixe2x80x94Ge with 1-2 xcexcm resist.
Accordingly, the need exists for improved techniques for fabricating optically smooth Sixe2x80x94Ge surfaces and for fabricating Sixe2x80x94Ge waveguides exhibiting a very low light propagation loss.
The present invention provides novel methods and techniques for etching Sixe2x80x94Ge, which are particularly useful for fabricating optically smooth Sixe2x80x94Ge surfaces.
In one embodiment of the present invention a novel etch technique is employed for etching Sixe2x80x94Ge, wherein SF6/hydrofluorocarbon etch chemistries are used at low bias power and wherein the etch technique is highly selective to organic photoresist. This etching technique results in optically smooth Sixe2x80x94Ge surfaces.
In another embodiment of the present invention an Sixe2x80x94Ge waveguide is fabricated by etching a cavity having optically smooth sidewall surfaces and an optically smooth bottom surface in a layer of a first Sixe2x80x94Ge composition, using SF6/hydrofluorocarbon plasma etch chemistry at low bias power, and then filling the cavity with a second Sixe2x80x94Ge composition that has a higher refractive index than the first Sixe2x80x94Ge composition. A cladding layer is subsequently deposited on the second Sixe2x80x94Ge composition that is formed in the cavity, thus fabricating the waveguide.
In a further embodiment of the present invention an Sixe2x80x94Ge waveguide is fabricated by etching a cavity having optically smooth sidewall surfaces and an optically smooth bottom surface in a first layer of a first Sixe2x80x94Ge composition, using SF6/fluorocarbon etch chemistries of the present invention, and then filling the cavity with a second Sixe2x80x94Ge composition that has a higher refractive index than the first Sixe2x80x94Ge composition. The top surface of the second Sixe2x80x94Ge composition is then etched, using SF6/hydrofluorocarbon etch chemistries of the present invention to provide an optically smooth top surface of the second Sixe2x80x94Ge composition that is deposited in the cavity. Subsequently, a second layer of the first Sixe2x80x94Ge composition is then deposited on the etched top surface of the second Sixe2x80x94Ge composition. A cladding layer is then formed on the second layer of the first Sixe2x80x94Ge composition. This technique results in a waveguide core having optically smooth top, side and bottom surfaces.
In another embodiment of the present invention a waveguide core is fabricated by subtractively etching a layer of a first Sixe2x80x94Ge composition that is deposited on an optically smooth first layer of a second Sixe2x80x94Ge composition wherein the first Sixe2x80x94Ge composition has a higher refractive index than the second Sixe2x80x94Ge composition. The subtractive etching technique of the present invention includes SF6/bydrofluorocarbon etch techniques of the present invention. A second layer of the second Sixe2x80x94Ge is deposited as a conformal layer on the core. Excess second layer material is removed from the core, and a top cladding layer is deposited on the core thereby forming the waveguide, wherein the core has optically smooth bottom and side surfaces.
In a further embodiment of the present invention a cluster tool is employed for executing processing steps of the novel techniques for fabricating Sixe2x80x94Ge waveguides of present invention. These processing steps include photoresist removal, Sixe2x80x94Ge etching, Sixe2x80x94Ge deposition and top cladding layer deposition, wherein the processing steps are carried out within the vacuum environment of the cluster tool.
In another embodiment of the present invention a manufacturing system is provided for fabricating Sixe2x80x94Ge waveguides of the present invention. This system includes a controller, such as a computer, that is adapted for interacting with a plurality of fabrication stations. Each of these fabrication stations performs a processing step that is utilized to fabricate the waveguides. Operative links provide connections between the controller and the fabrication stations. A data structure, such as a computer program, causes the controller to control the processing steps which are performed at the fabrication stations.